Hybrid polarizing beam splitter

ABSTRACT

Polarizing beam splitters and systems incorporating such beam splitters are described. More specifically, hybrid polarizing beam splitters and systems with such beam splitters that incorporate polymeric reflective polarizers aligned with MacNeille or wire grid reflective polarizers are described.

FIELD

This disclosure generally relates to polarizing beam splitters such asthose used in optical projectors.

BACKGROUND

A projection system typically includes a light source, one or more imageforming components or imagers, projection optics, and potentially ascreen. Often, imagers used in projection systems arepolarization-rotating, image-forming devices, such as liquid crystaldisplay imagers, which operate by rotating the polarization of the lightto produce an image corresponding to digital video signals. Imagers usedin projection systems typically rely on polarizers to separate lightinto a pair of orthogonal polarization states (e.g., s-polarization andp-polarization) before the light may be imaged. Therefore, projectionsystems will also generally include a polarizing beam splitter (PBS) toserve this purpose.

Due to new demands on PBSs, in part due to their new uses inapplications such as, e.g., three-dimensional projection orultra-compact projection systems that have relatively high light output,a number of new issues have arisen. The present application providesarticles and systems that address such issues.

SUMMARY

In one aspect, the present description relates to an optical componentincluding a first and second reflective polarizer arranged so that lighthaving a first polarization state passes through each reflectivepolarizer and light having a second polarization state reflects fromeach reflective polarizer and where the first reflective polarizer is apolymeric reflective polarizer and the second reflective polarizer is aMacNeille reflective polarizer or a wire grid reflective polarizer. Thepolymeric reflective polarizer may contain alternating layers ofpolymeric film having different refractive indices. The secondreflective polarizer may be a MacNeille polarizer made by depositinginorganic dielectric layers onto optical glass. In some embodiments, thefirst polarization state is a linear polarization state and the secondpolarization state may be a linear polarization state orthogonal to thefirst polarization state. An optically clear adhesive may be positionedbetween the first and second reflective polarizers. The optically clearadhesive may be a pressure sensitive adhesive. In one embodiment, theoptical component is a pellicle component. In an alternative embodiment,the optical component contains a first and second prism with the firstand second reflective polarizers between diagonal faces of the first andsecond prisms.

In another aspect, the present description relates to a system includingthe optical component described previously and further including a lightsource that directs light toward the optical component with lightintercepting the first reflective polarizer at an angle of about 45degrees. Light is reflected from the first reflective polarizer towardsa polarization rotating reflector which reflects light back through thefirst and second reflective polarizer onto a reflective imager whichimages the light and reflects imaged light toward the second reflectivepolarizer. The imaged light intercepts the second reflective polarizerat an angle of about 45 degrees and exits the system. The polarizationrotating reflector may include a broadband mirror and a quarter waveplate adjacent to the broadband mirror. The reflective imager may be aLiquid Crystal on Silicon (LCoS) imager. The effective pixel resolutionof the imaged light may be less than 12 microns and may be less than 6microns. The system may include a post-polarizer and/or a projectionlens.

In another aspect, the present description relates to a system includingthe optical component described previously and further including a lightsource that directs light toward the optical component with lightintercepting the first reflective polarizer at an angle of about 45degrees. Light is reflected from the first reflective polarizer towardsa reflective imager which images the light and reflects imaged lightthrough the first and second reflective polarizer. The imaged light maypass through a projection lens before exiting the system.

In another aspect, the present description relates to a system includingthe optical component described previously and further including a lightsource that directs light toward the optical component with lightintercepting the second reflective polarizer at an angle of about 45degrees.

Light is reflected from the second reflective polarizer towards areflective imager which images the light and reflects imaged lightthrough the second and first reflective polarizer. The imaged light maypass through a projection lens before exiting the system.

In another aspect, the present description relates to an opticalcomponent containing a first and second prism and containing a first andsecond reflective polarizer arranged so that light having a firstpolarization state passes through each reflective polarizer and lighthaving a second polarization state reflects from each reflectivepolarizer. The first reflective polarizer is a polymeric reflectivepolarizer and the second reflective polarizer is a MacNeille reflectivepolarizer or a wire grid reflective polarizer. The first and secondreflective polarizers are disposed between a first diagonal surface ofthe first prism and a second diagonal surface of the second prism. Thesecond reflective polarizer may be a MacNeille polarizer made bydepositing inorganic layers onto the second diagonal surface. In someembodiments, an optically clear adhesive may be positioned between thefirst reflective polarizer and the first prism and/or between the firstreflective polarizer and the second reflective polarizer. The opticallyclear adhesive may be a pressure sensitive adhesive.

In another aspect, the present description relates to a systemcontaining the optical component having prisms described previously andfurther including a light source that directs light toward the opticalcomponent with light intercepting the first reflective polarizer at anangle of about 45 degrees. Light is reflected from the first reflectivepolarizer towards a polarization rotating reflector which reflects lightback through the first and second reflective polarizer onto a reflectiveimager which images the light and reflects imaged light toward thesecond reflective polarizer. The imaged light intercepts the secondreflective polarizer at an angle of about 45 degrees and exits thesystem. The imaged light may pass through a projection lens beforeexiting the system. The effective pixel resolution of the imaged lightmay be less than 12 microns and may be less than 6 microns. The systemmay include a post-polarizer positioned adjacent to the second prism.

In another aspect, the present description relates to a system includingthe optical component having prisms described previously and furtherincluding a light source that directs light toward the optical componentwith light intercepting the second reflective polarizer at an angle ofabout 45 degrees. Light is reflected from the second reflectivepolarizer towards a reflective imager which images the light andreflects imaged light through the second and first reflective polarizer.The imaged light may pass through a projection lens before exiting thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,wherein:

FIG. 1 illustrates a cross-sectional schematic of components used in anoptical system;

FIG. 2 illustrates a cross-sectional schematic of a polarizing beamsplitter;

FIG. 3 illustrates a cross-sectional schematic of a polarizing beamsplitter;

FIG. 4 illustrates a cross-sectional schematic of components used in anoptical system.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe foregoing specification and attached claims are approximations thatcan vary depending upon the desired properties sought to be obtained bythose skilled in the art utilizing the teachings disclosed herein.

A high performance polarizing beam splitter (PBS) is essential forcreating a viable optical engine for a projector that uses LiquidCrystal on Silicon (LCoS) imagers. In addition, a PBS may be requiredeven for nominally unpolarized imagers such as DLP imagers when suchimagers are required to image polarized light. Typically, a PBS willtransmit nominally p-polarized light and reflect nominally s-polarizedlight. A number of different types of PBSs have been used in opticalengines, including MacNeille type PBSs and wire grid polarizers.However, PBSs utilizing polymeric reflective polarizers have proven tobe one of the most effective PBSs for issues associated with lighthandling in projection systems, including the ability to effectivelypolarize over a range of wavelengths and angles of incidence and withhigh efficiencies both in reflection and transmission. Such polymericreflective polarizers may be multilayer optical films (MOF) made by 3MCompany (St. Paul, Minn.), as described in U.S. Pat. No. 5,882,774 toJonza et al., and U.S. Pat. No. 6,609,795 to Weber et al. Use of MOFbased PBSs in optical engines results in significant improvements inboth optical efficiency and in contrast compared with MacNeille type orwire grid PBSs.

With the advent of a number of new imaging and projection applications,including, e.g., three-dimensional projection and imaging, newchallenges have arisen. Specifically, in at least some applications, itmay be required that a PBS provide imaged light where the image qualityis maintained not only when transmitted through a reflective polarizingfilm, but also when reflected by a reflective polarizing film.Unfortunately, polarizers based on multilayer optical film, despitetheir other major advantages, may be difficult to formulate with therequisite flatness to reflect imaged light with sufficiently lowdistortion. However, the concerns of effectively polarizing a wide arrayof angles of incident light and wavelengths of incident light must stillbe addressed. It would therefore be highly desirable to provide apolarizing beam splitter that has the benefits of a PBS that containsmultilayer optical film, while also achieving low distortion for imagedlight reflected off of the PBS towards a viewer or screen. In oneaspect, the present description provides such a solution.

In another aspect, the present description provides a PBS having a longlifetime and providing a high contrast. When light interacts with amaterial, such as a polymeric reflective polarizer, it can cause damagethat degrades the performance of the material. Experiments have shownthat blue light causes the most damage in polymeric reflectivepolarizers and that the rate of damage caused by the blue light dependson the energy density of blue light incident on the polymeric reflectivepolarizer. The energy flux may be expressed in units of, for example,W/mm² and the total dose may be expressed in units of, for example,MJ/mm².

Accordingly, polymeric reflective polarizers used in polarizing beamsplitters within projection systems degrade after a given amount oftime. This degradation becomes far more rapid with higher intensitylight sources, such that where two films have been exposed to equivalentdoses of light (MJ/mm²), the film that has been exposed to these dosesat higher intensity will degrade more quickly. This degradation maygenerally cause a “yellowing” of the light that travels through or isreflected off of the reflective polarizer. In one aspect, the presentdescription addresses this problem by providing a projection system thatis capable of exposure to high intensity and doses of incident lightwhile maintaining necessary performance over a longer lifetime andmaintaining the high contrast ratio provided by MOF based PBSs.

FIG. 1 provides an illustration of one optical system according to thepresent description. Optical system includes an imager 131. In a numberof embodiments, such as that illustrated in FIG. 1, the imager will bean appropriate reflective imager. Often, imagers used in projectionsystems are polarization-rotating, image-forming devices, such as liquidcrystal display imagers, which operate by rotating the polarization ofthe light to produce an image corresponding to digital video signals.Such imagers, when used in projection systems, typically rely onpolarizers to separate light into a pair of orthogonal polarizationstates (e.g., s-polarization and p-polarization) before light isincident upon the imager. Two common imagers that may be used in theembodiment shown in FIG. 1 include a liquid crystal on silicon (LCoS)imager, or digital light processing (DLP) imager. Those skilled in theart will recognize that the DLP system will require some modification tothe illumination geometry as well as an external means of rotating thepolarization (such as a retarder plate) in order to make use of the PBSconfiguration shown in FIG. 1.

The optical system in FIG. 1 also includes a polarizing beam splitter(PBS) 101. Within PBS 101 is a hybrid reflective polarizer 122 thatincludes a first reflective polarizer 125, which will generally be apolymeric reflective polarizer and a second reflective polarizer 126,which will typically be a MacNeille or wire grid polarizer. Thepolymeric reflective polarizer may be a multilayer optical film such asthose available from 3M Company (St. Paul, MN) and described in, e.g.,U.S. Pat. No. 5,882,774 to Jonza et al., and U.S. Pat. No. 6,609,795 toWeber et al., each of which is hereby incorporated by reference in itsentirety. In the embodiment shown in FIG. 1, the PBS includes firstprism 102 and second prism 103 on opposite sides of the hybridreflective polarizer. The first reflective polarizer 125 of hybridreflective polarizer 122 is positioned adjacent first diagonal surface112 of first prism 102 and the second reflective polarizer 126 ispositioned adjacent second diagonal surface 113 of second prism 103. AMacNeille reflective polarizer contains multiple inorganic dielectriclayers. In one embodiment, reflective polarizer 126 is a

MacNeille reflective polarizer made by depositing inorganic dielectriclayers directly onto second diagonal surface 113 of second prism 103.

A reflective polarizer generally reflects light having a firstpolarization state and transmits light having a second polarizationstate. In hybrid PBS 101, the first (polymeric) reflective polarizer 125and the second (MacNeille or wire grid) reflective polarizer 126 aredisposed so that light having a first polarization state is reflectedfrom both reflective polarizers 125, 126 and so that light have a secondpolarization state is transmitted through both reflective polarizers125, 126. In some embodiments, the first polarization state is a linearpolarization state and in some embodiments the second polarization stateis a linear polarization state orthogonal to the first polarizationstate.

The optical system of FIG. 1 also includes a polarization rotatingreflector 135. The polarization rotating reflector 135 may include abroadband mirror 139 and a retarder 137 positioned between the broadbandmirror 139 and the first prism 102 (as illustrated in FIG. 1).Polarization rotating reflectors (such as element 135) are discussedelsewhere, for example, in U.S. Patent Application No. 2011/0007392(English et al.), the relevant portions of which are hereby incorporatedby reference. A polarization rotating reflector reverses the propagationdirection of the light and alters the magnitude of the polarizationcomponents, depending on the components within the polarization rotatingreflector and their orientation with respect to one another. Thepolarization rotating reflector may, in a number of embodiments, includea reflector and a retarder. In one embodiment, the reflector can be abroadband mirror that blocks the transmission of light by reflection.The retarder can provide any desired retardation, such as an eighth-waveretarder, a quarter-wave retarder, and the like. In embodimentsdescribed herein, there may be an advantage to using a quarter-waveretarder and an associated reflector. Linearly polarized light ischanged to circularly polarized light as it passes through aquarter-wave retarder aligned at an angle of 45° with respect to theaxis of light polarization. Reflections from the reflective polarizerand quarter-wave retarder/reflectors result in efficient light outputfrom the PBS. In contrast, linearly polarized light is changed to apolarization state partway between s-polarization and p-polarization(either elliptical or linear) as it passes through other retarders andorientations, and can result in a lower efficiency of the PBS.

Referring to FIG. 1, light source 155, which may be a solid-stateemitter such as a laser or a light emitting diode (LED) and may includea pre-polarizer (not shown), emits light 153 having an s-polarizationstate toward hybrid PBS 101. Light 153 passes through first prism 102,is incident upon first reflective polarizer 125 (here again, a polymericreflective polarizer) at an angle of about 45 degrees, is reflectedtoward polarization rotating reflector 135, intercepts polarizationrotating reflector 135 at an angle of about 90 degrees, and is thenreflected back towards hybrid reflective polarizer 122 as p-polarizedlight 155. P-polarized light 155 passes through hybrid reflectivepolarizer 122, is incident on imager 131 at an angle of about 90degrees, is imaged, and is reflected back towards the hybrid reflectivepolarizer 122 as s-polarized imaged light 157 (as the imager 131 rotatespolarization state of the reflected light by 90 degrees). S-polarizedlight 157 intercepts the second polarizer 126 (a MacNeille or wire gridpolarizer) at an angle of about 45 degrees, is reflected, interceptsoptional post-polarizer 195 and projection lens 142 at an angle of about90 degrees and exits the system. Optional post-polarizer 195 may beincluded to remove any p-polarized light that reflects from the hybridreflective polarizer after reflecting from the imager in the off state.

Because imaged light 157 is reflected from a MacNeille or wire gridpolarizer (second polarizer 126), any deviation from flatness of theMacNeille or wire grid polarizer would tend to distort the imaged light.A surface is said to be optically flat if it is sufficiently flat thatimages reflected from the surface are not significantly distorted.Fortunately, both MacNeille polarizers and wire grid polarizers aretypically optically flat. This is typically not the case for MOF basedreflective polarizers unless special techniques are used to make the MOFfilm optically flat, such as those techniques discussed in commonlyowned and assigned U.S. Provisional Pat. App. No. 61/564161. So if theMacNeille or wire grid polarizer 126 were removed from the PBS in FIG.1, it would be difficult to achieve the desired low distortion image.The deviation from optical flatness can be characterized by theeffective pixel resolution of the PBS, which is defined as the maximumresolution that can be expected to be reliably (across 95% of the image)resolved after an image is reflected from the particular PBS. Mostcurrent imagers (LCoS and DLP) have a pixel size range from about 12.5μm down to around 5 μm. So in order to be useful in a reflective imagingsituation, it is desirable for the PBS be able to resolve down to atleast about 12.5 um, and ideally better. Therefore it is preferable thatthe effective pixel resolution of a PBS used in a reflective imagingsituation be no more than about 12.5 um, and preferably lower. Thiswould be considered a high effective resolution. A method for measuringthe effective resolution is described in U.S. Pat. App. No. 61/564,161,the relevant portions of which are hereby incorporated herein byreference. Conventional MacNeille or wire grid reflective polarizers aretypically sufficiently optically flat that they are capable ofreflecting light with an effective pixel resolution of less than 12microns, less than 9 microns, or less than 6 microns.

To illustrate the contrast and efficiency benefits of the hybrid PBSs ofthe present disclosure, MOF and MacNeille reflective polarizers weremodeled using the coefficients of reflection (Rs and Rp for s and ppolarization, respectively) and the coefficients of transmission (Ts andTp for s and p polarization, respectively) in Table I.

TABLE I MOF MacNeille Rs 0.99 0.92 Ts 0.001 0.004 Rp 0.025 0.1 Tp 0.970.9

The efficiency of the system shown in FIG. lwas estimated asRs^((a))Tp^((a))T_(p) ^((b))Rs^((b)) times 100% where the superscript(a) refers to the polymeric reflective polarizer 125 and the superscript(b) refers to the MacNeille or wire grid based reflective polarizer 126.For comparison, the efficiencies of single film reflective polarizerswere estimated as Rs Tp Rs times 100%.

The contrast ratio for the system shown in FIG. 1 without apost-polarizer was estimated as the efficiency divided by the percentageof incident light transmitted through the system with the imager in theoff state. For the hybrid reflective polarizer, this percentage wasestimated as Ts^((a))Ts^((b))+Rs^((a))Tp^((a))Tp^((b))Rp^((b)) times100%, where the first term accounts for incident s-polarized light thatpasses directly through the PBS and the second term accounts forp-polarized light that reflects from the reflective polarizer afterbeing reflected from the imager in the off state. For a single filmreflective polarizer, the corresponding percentage was estimated asTs+Rs Tp Rp times 100%. For systems with a post polarizer, thepercentage of incident light transmitted through the system with theimager in the off state was estimated as Ts^((a))Ts^((b)) times 100% forthe hybrid reflective polarizer and estimated as Ts times 100% for asingle film reflective polarizer. Table II shows the results of thisanalysis for a hybrid polarizer consisting of a MacNeille and an MOFbased reflective polarizer and for MacNeille and MOF based reflectivepolarizers alone. From this table it can be seen that the hybridreflective polarizer has a higher efficiency and a higher contrast thanthe MacNeille reflective polarizer. It can also be seen that when thepost-polarizer is included, the hybrid PBS provides a higher contrastthan either of the single reflective polarizer based PBSs.

TABLE II MacNeille MOF Hybrid Only Only Efficiency 80% 76% 95% Contrastratio without 9.2 8.8 38 post-polarizer Contrast ratio withpost-polarizer 199000 190 951

FIG. 2 shows a PBS according to one embodiment of the presentdescription. The hybrid PBS 201 can be constructed by depositingdielectric layers 226 onto a first diagonal surface 212 of a first prism202 and then laminating a first polymeric reflective polarizer 225 tothe dielectric layers 226 using a first optically clear adhesive (OCA)layer 228. The dielectric layers 226 may act as a second reflectivepolarizer. A second prism 203 may be laminated to the polymericreflective polarizer 225 opposite the first prism using a second OCAlayer 223 adjacent second diagonal surface 213. The first or secondprism, 202 or 203, may be constructed from optical glass or from asuitable plastic which may include polymethyl methacrylate (PMMA),cyclic olefins (CO), copolymers of PMMA and CO, and those materialsdiscussed in commonly owned U.S. Pat. No. 7,529,029, column 16, lines44-54. This section of U.S. Pat. No. 7, 529,029 is hereby incorporatedby reference. The OCA layers, 228 or 223, can be any suitable adhesivethat does not significantly affect the transmission of visible light.The OCA layers, 228 or 223, may in some embodiments bepressure-sensitive adhesives. In other embodiments, the OCA layers, 228or 223, may, without limitation, be photocurable adhesives orthermally-cured adhesives or two-part adhesives.

An alternative embodiment of the PBS of the present disclosure is thepellicle design shown in FIG. 3 where the hybrid PBS 301 is a freestanding element rather than adhered or otherwise attached to and/orpositioned between prisms. In this embodiment, inorganic dielectriclayers 326 are deposited onto a substrate 304, which may be an opticalglass. A first reflective polarizer 325 (in this case, a polymericreflective polarizer) is then laminated to the dielectric layers 326using an optically clear adhesive layer 328. In this embodiment, onceagain, the dielectric layers may act as a second reflective polarizer.The OCA layer 328 may be any of the OCAs previously described. In aprism design such as shown in FIGS. 1-2, it is in some cases desirableto apply an anti-reflective coating to the outer surfaces of the prismsin order to reduce reflection losses in the system. A pellicle designsuch as shown in FIG. 3, allows for a thinner PBS with less opticalsurface area for any anti-reflective coating that may be desired. Thus,in some cases, a pellicle design may be the most economical choice.

As noted previously, the polymeric materials used in MOF based PBSs tendto degrade after extended use with high intensity light sources. FIG. 4provides an illustration of one optical system according to the presentdescription which addresses this problem. The highest intensity light inFIG. 4 is the s-polarized light 453 which is incident on the MacNeilleor wire grid layer 426 of the hybrid reflective polarizer 422 at anangle of about 45 degrees. MacNeille or wire grid layer 426 may also beunderstood as a “second reflective polarizer.” S-polarized light 453 isreflected from hybrid reflective polarizer 422 towards optical imager431. S-polarized light 453 intercepts optical imager 431 at an angle ofabout 90 degrees, is imaged and is reflected toward hybrid reflectivepolarizer 422 as p-polarized imaged light 457, which passes throughMacNeille or wire grid reflective polarizer (or second reflectivepolarizer) 426 and through polymeric reflective polarizer 425. Polymericreflective polarizer 425 may also be understood for purposes of thisdescription as a “first reflective polarizer.” P-polarized imaged light457 intercepts projection lens 442 at an angle of about 90 degrees andexits the system. The average intensity of the p-polarized imaged light457 is significantly less than that of s-polarized light 453 since onlythe pixels in the “on-state” contribute to the p-polarized imaged light457. The polymeric reflective polarizer 425 is exposed only to therelatively low intensity p-polarized imaged light 457 and thus has ahigher lifetime than it would have without the MacNeille or wire gridpolarizer 426.

The efficiency of the system shown in FIG. 4 was estimated asRs^((a))Tp^((a))Tp^((b)) times 100% where here the superscript (a)refers to the MacNeille or wire grid based reflective polarizer (secondreflective polarizer) 426 and the superscript (b) refers to thepolymeric reflective polarizer (first reflective polarizer) 425. Forcomparison, the efficiency of single film reflective polarizers wereestimated as Rs Tp times 100%. Since the efficiency is the percentage ofincident light transmitted through the system with the imager in the onstate, the contrast for the system shown in FIG. 4 can be estimated asthe efficiency divided by the percentage of incident light transmittedthrough the system with the imager in the off state. For the hybridreflective polarizer, this percentage was estimated asRs^((a))Ts^((a))Ts^((b)) times 100%. For a single film reflectivepolarizer, the corresponding percentage was estimated as Rs Ts times100%.

Parameters from Table I were used to produce the results shown in theTable III. It can be seen that although the efficiency is slightly lowerfor the hybrid polarizer compared to the MacNeille polarizer, thecontrast is much higher. For some applications, the high contrast of thehybrid polarizer combined with the higher lifetime of the hybridpolarizer compared to the MOF polarizer make the hybrid polarizer thepreferred choice.

TABLE III MacNeille MOF Hybrid Only Only Efficiency 80% 83% 96% Contrastratio 218000 225 960

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations can besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present disclosure. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisdisclosure be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. An optical system comprising a hybrid reflectivepolarizer comprising adjacent first and second reflective polarizers,such that in a same wavelength range, the first and second reflectivepolarizers reflect light having a same first polarization state andtransmit light having a same second polarization state substantiallyorthogonal to the first polarization state, the first reflectivepolarizer comprising inorganic dielectric layers or a wire grid, thesecond reflective polarizer comprising polymeric layers, such that whenan incident light of the optical system in the wavelength range isincident on the hybrid reflective polarizer, the incident light is firstincident on the first reflective polarizer if the incident light has thefirst polarization state and first incident on the second reflectivepolarizer if the incident light has the second polarization state. 2.The optical system of claim 1 further comprising a polarization rotatingreflector making an oblique angle with the hybrid reflective polarizer.3. The optical system of claim 1, wherein the second reflectivepolarizer comprises alternating polymeric layers having differentrefractive indices.
 4. The optical system of claim 1, wherein the firstreflective polarizer comprises inorganic dielectric layers.
 5. Theoptical system of claim 1, wherein the first reflective polarizercomprises a wire grid.
 6. The optical system of claim 1, wherein thefirst polarization state is a linear polarization state.
 7. The opticalsystem of claim 1, wherein the second polarization state is a linearpolarization state.
 8. The optical system of claim 1 further comprisingan adhesive disposed between the first and second reflective polarizers.